A fuel cell simultaneously generates electricity and heat by an electrochemical reaction of hydrogen-containing fuel gas, and oxygen gas such as the air containing oxygen. An example of a single cell of a polymer electrolyte fuel cell is schematically shown in FIG. 13. The left diagram of FIG. 13 shows arrangement of its respective constituent elements before being laminated and the right diagram of FIG. 13 shows a laminated state of these elements. A single cell 1 is constituted by an electrolyte membrane 1a, and a pair of electrodes (an air electrode 1b and a fuel electrode 1c) sandwiching the electrolyte membrane 1a from both sides. Bipolar plates 2 have channel-formed surfaces 2b, 2c on which a plurality of channels are formed. The bipolar plates 2 are housed in resin bipolar plate frames 3 and laminated so that the air electrode 1b and the channel-formed surface 2b face each other and the fuel electrode 1c and the channel-formed surface 2c face each other. Thus, gas flow passages sectioned by electrode surfaces and the channels are formed between the electrodes and the bipolar plates, and efficiently supply fuel gas and oxygen gas, which are reaction gases of the fuel cell, to the electrode surfaces.
In the fuel cell, fuel gas and oxygen gas need to be separately supplied to the entire electrode surfaces without being mixed with each other. Therefore, the bipolar plates need to be gas tight. Furthermore, the bipolar plates need to have a good electric conductivity in order to collect electrons generated by reaction and serve as electric connectors for connecting adjoining single cells when a plurality of single cells are stacked. Moreover, because polymer electrolyte membrane surfaces are strongly acidic, the bipolar plates are demanded to have corrosion resistance.
Therefore, as a bipolar plate material, it is common to use graphite plates. However, since the graphite plates break easily, the graphite plates have a problem with workability in producing bipolar plates by forming a plurality of gas passages thereon, flattening the surfaces and so on. On the other hand, because metallic materials have good workability as well as good electric conductivity and especially titanium and stainless steel have good corrosion resistance, the metallic materials can be used as bipolar plate materials. However, since metallic materials having good corrosion resistance are easily passivated, the metallic materials have a problem of increasing internal resistance of a fuel cell and causing a voltage drop.
PTL 1 discloses a fuel cell bipolar plate comprising a metal member and having gold directly plated on a surface to contact an electrode of a single cell. Because the surface to contact an electrode is plated with gold, contact resistance between the bipolar plate and the electrode decreases, electric conductivity improves and output voltage of the fuel cell increases. It is believed that if a bipolar plate is fully covered with an electrically conductive material such as corrosion-resistant gold plating, the metal member becomes resistant to corrosion and metal ions are prevented from being eluted away and the metal member exhibits low contact resistance. However, gold plating constitutes a major obstacle to practical use in view of costs and resources.
PTL 2 discloses stainless steel having high corrosion resistance and low electric contact resistance. Specifically, electrically conductive metallic inclusions are dispersed so as to break through a passivated film on a surface of stainless steel, thereby decreasing contact resistance of the stainless steel. However, even if a matrix is passivated, elution of chromium ions, iron ions and the like, which is a cause of degradation of an electrolyte membrane, is inevitable. Moreover, because the metallic inclusions also gradually corrode under potential-applied highly corrosive environments, it is anticipated that if these metallic inclusions are passivated by corrosion, output voltage decreases.
In order to solve such problems as above, attention is drawn to a fuel cell bipolar plate formed by coating a surface of a metal substrate with an electrically conductive amorphous carbon film.
For example, each of PTL 3 and PTL 4 discloses a bipolar plate formed by coating a metal plate with an electrically conductive amorphous carbon film. Upon coating the metal plate with the amorphous carbon film, the bipolar plate exhibits corrosion resistance. Particularly in PTL 4, an amorphous carbon film mainly comprising carbon and hydrogen is formed on a surface of a metal substrate by using ionized vapor deposition. PTL 4 states that film formation by ionized vapor deposition generates an intermediate layer in which an incident carbon film-forming molecule and atoms constituting a surface layer of the substrate are chemically bonded together.
By the way, PTL 5 secures electric conductivity of an amorphous carbon film by increasing the amount of carbon having an sp2 hybrid orbital and decreasing the content of hydrogen. Depending on a difference in atomic orbital in chemical bonding, carbon atoms are classified into three kinds: carbon having an sp hybrid orbital (Csp), carbon having an sp2 hybrid orbital (Csp2) and carbon having an sp3 hybrid orbital (Csp2). For example, diamond, which consists only of Csp2, forms only σ bonds and exhibits high electrical insulation due to localization of σ electrons. On the other hand, graphite consists only of Csp2, forms σ bonds and π bonds, and exhibits high electric conductivity due to delocalization of π electrons. In the amorphous carbon film recited in PTL 5, delocalization of π electrons is promoted by a high ratio of Csp2 in the entire carbon, and molecular termination by C—H bonds (σ bonds) is suppressed by a decrease in the hydrogen content. As a result, the amorphous carbon film recited in PTL 5 exhibits a high electric conductivity.